Bimodal Volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and Tectonic Implications of Age-Progressive Rhyolites

Article Volume 14, Number 8 8 August 2013 doi: 10.1002/ggge.20175 ISSN: 1525-2027

Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites

Mark T. Ford, Anita L. Grunder, and Robert A. Duncan College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon, 97331, USA ([email protected] or [email protected])

[1] Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO2 >72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40Ar/39Ar incremental heating ages on rhyolites, and establish that the timing of the age-progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and northwestern-most Basin and Range (NWBR) rhyolite volcanism into a single age-progressive trend. The spatial-temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment-ﬁlled basins within the more volcanically active HLP. The west- northwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi-stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterﬂow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR.

Components: 14,332 words, 7 ﬁgures, 2 tables. Keywords: high lava plains; bimodal volcanism; intraplate volcanism; Ar-Ar geochronology; Oregon geology; slab rollback. Index Terms: 1115 Radioisotope geochronology: Geochronology; 1033 Intra-plate processes: Geochemistry; 1031 Sub- duction zone processes: Geochemistry; 3613 Subduction zone processes: Mineralogy and Petrology; 3615 Intra-plate processes: Mineralogy and Petrology; 8170 Subduction zone processes: Tectonophysics; 3060 Subduction zone processes: Marine Geology and Geophysics; 8415 Intra-plate processes: Volcanology; 8413 Subduction zone processes: Volcanology. Received 30 January 2013; Revised 6 May 2013; Accepted 14 May 2013; Published 8 August 2013.

Ford, M. T., A. L. Grunder, and R. A. Duncan (2013), Bimodal volcanism of the High Lava Plains and northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites, Geochem. Geophys. Geosyst., 14, 2836–2857, doi:10.1002/ggge.20175.

1. Introduction this tectonically complicated region, and contain multiple episodes of Oligocene and younger volca- [2] The late Cenozoic tectonomagmatic evolution nism. Over the past 12 Ma, silicic volcanism has of the Paciﬁc Northwestern United States is com- produced a WNW age-progressive trend across the plex, involving three geodynamic mantle features: HLP-NWBR, nearly opposite from the trend of co- the Yellowstone hotspot, mantle processes related eval age-progressive rhyolites of the Yellowstone- to the subduction of the Cascadia slab, and exten- Snake River Plain system [MacLeod et al., 1976; sion of the Basin and Range. These tectonic- Jordan et al., 2004]. In contrast, basaltic volca- magmatic elements contribute to the formation of nism over the same period has not left a systematic the largest Cenozoic continental volcanic province temporal pattern in either the HLP-NWBR or in the world, including the Columbia River Yellowstone-Snake River Plain provinces [cf., Basalts, Steens Basalts, volcanic provinces of the Kuntz et al., 1992; Jordan et al., 2004]. Most Yellowstone—Snake River Plain track, Owyhee workers attribute the HLP age-progressive rhyo- Plateau and High Lava Plains (HLP), and lites to lithospheric processes such as extension northwestern-most Basin and Range (NWBR) [Fitton et al., 1991; Christiansen et al., 2002] or (Figure 1) [Smith and Luedke, 1984; Pierce and extension coupled with crustal block rotation Morgan, 1992; Hooper et al., 2002; Shoemaker [Carlson and Hart, 1987] or to a westward spread and Hart, 2002]. This region is a modern example of a portion of the deep-seated Yellowstone plume of intracontinental magmatic modiﬁcation of the head, with or without involvement of Cascadia lithosphere. slab subduction [Draper, 1991; Camp and Ross, [3] The HLP and NWBR physiographic provinces 2004; Jordan et al., 2004]. Here we report 18 new of Oregon (Figure 1) are prominent elements of 40Ar/39Ar age determinations for rhyolitic

Figure 1. Regional tectonic setting of the High Lava Plains (outlined in red) and surrounding provinces. The Northwest Basin and Range refers to that portion of the Great Basin that extends into southern Oregon. Dashed line indicates approximate position of the 87Sr/86Sr 0.706 line [after Kistler and Peterson, 1978], representing the western edge of the Proterozoic or older craton of North America with generally Mesozoic and younger accreted terranes to the west. Lightly shaded region indicates middle Mio- cene and younger extensional normal faulting (Basin and Range style) [after Pezzopane and Weldon, 1993]. Two sets of arrows give the absolute plate motions and velocities based on the NUVEL-1A model of DeMets, et al. [1994] and the HS3-NUVEL-1A model of Gripp and Gordon [2002]. North American Plate motion is between 19 and 26 km/Ma in central Oregon, near the center of the studied area (outlined by blue dashed box) and the Juan de Fuca Plate ranges from 16 to 19 km/Ma. Physiographic provinces adapted after Dicken [1950]; Walker and MacLeod [1991]; MacLeod, et al. [1995]; Shoemaker and Hart, [2002].

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Figure 2. Slopeshade DEM and fault map of south-central and southeastern Oregon. Major, generally NNE trending Basin and Range type faults are shown with thick lines. Thin, generally NW trending lines obliquely cutting the HLP represent the Brothers Fault Zone while the NWBR is similarly cut but the Eugene—Denio Fault Zone. Heavy red dashed line outlines the HLP physiographic province as described in the text and Figure 1. The HLP has generally muted terrain (lighter shading) compared to surrounding provinces. Figure is adapted from the Oregon State Geologic Map [Walker and MacLeod, 1991] and simpliﬁed from the spatial database of Miller et al. [2003]. samples, mostly from the NWBR, and further concentrated study (e.g., Glass Buttes). The recent document the distribution of westward age- geophysical and geological investigation of the progressive rhyolites within Oregon. We also pro- HLP has sparked a comprehensive body of work, vide a comprehensive, quantitative volume esti- much of which is distilled in this Geochemistry mates for volcanic units in this bimodal province Geophysics Geosystems volume. which has erupted nearly equal volumes of basalt- basaltic andesite and rhyolite over the past 12 Ma. [5] The physiographic HLP (Figure 1) generally Cascadia slab plate-driven asthenospheric ﬂow is comprises a late Miocene and younger volcanic the driving force of our model for volcanism in the province that is approximately 90 km wide and HLP and NWBR and no plume is required. 275 km long extending across eastern and central Oregon. It is bounded by the Owyhee Plateau, Cascades volcanic arc, extending Basin and Range 1.1. Physiographic and Tectonic (the NWBR) and to the north by the relatively Background of the HLP-NWBR unextended, accreted terranes of the Blue Moun-

[4] Since the original reconnaissance mapping sur- tains region. Generally, late Miocene and younger veys of Russell [1884, 1905], there has been per- flat-lying basaltic lava flows, ignimbrites and vol- sistent interest in the igneous rocks and tectonics caniclastic sediments cover much of the muted to- of the HLP-NWBR region [e.g., Williams, 1935; pography that characterizes the HLP (Figure 2). Green, 1973; Wells, 1979; Carlson and Hart, Oligocene through middle Miocene silicic vol- 1987; Draper, 1991; Johnson and Ciancanelli, canic rocks are also sparsely exposed within the HLP [MacLeod et al., 1976; McKee and Walker, 1984; Johnson and Grunder, 2000; Jordan et al., 1976; Jordan et al., 2004] (Figures 3a and 3b). 2004; Boschmann, 2012]. Much of the work over the past three decades is contained in university [6] The HLP is cut obliquely by the Brothers Fault student theses with a few specific areas receiving Zone, which is characterized by small, northwest-

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Figure 3. Map of the distribution of central and eastern Oregon rhyolites and basalts shows the spatial distribution of silicic lavas and tuffs though time with (a) John Day age (37–20 Ma), (b) Steens age (17.5–15 Ma), (c) Deschutes age (7–6 Ma), and (d) age- progressive HLP and NWBR <12 Ma province. Lettered, outlined, areas on Figure 3d indicate units that differ from the Walker and MacLeod [1991] Oregon State geologic map as follows: (A) basalt of Dry Mountain (originally mapped as an andesite, Iademarco [2009]), (B) basalt or basaltic andesite with local hydrothermal silica sinter, (C) basalt or basaltic andesite, (D) basalt or basaltic andesite. Units B, C,andD were originally mapped as rhyolite. HLP volcanic province extends east of the physiographic HLP and includes Duck Creek Butte (Duck Butte Eruptive Center) and Jordan Craters. Proposed location for the Rattlesnake Tuff source [Streck and Grunder, 1997] given and proposed locations of vents, based in part from this map and ﬁeld evidence (Grunder, unpublished) for other tuffs (Hampton, Prater Creek and Devine Canyon) shown with dashed circles. Black lines are major roads; heavy black dashed line outlines the HLP physiographic province from Figure 1. Green boxes are locations of major towns including Bend, Burns, and Lakeview. Many of the locations mentioned in the text are given and all four maps cover the same region. 2839 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175

Table 1. New 40Ar/39Ar Age Determinations (in Ma) From This Studya

Accepted Age wt% K2Oif (Plateau) 2 2 % Radio-Genic Inverse 2 Sample Number Local Name Typeb WR Age Error percent Ar 40 Isochron Age Error

MTF 07-04 Glass Buttes ‘‘A’’ WR 4.20 5.79 0.04 0.75 98.49 5.74 0.11 MTF 07-05 Glass Buttes ‘‘E’’ WR 4.48 6.49 0.05 0.80 100 6.49 0.06 MTF 07–38 North Connelly Hills WR 3.72 5.59 0.05 0.86 77.7 5.59 0.07 MTF 07–16 Cougar Mt (Lake Co.) WR 3.82 4.34 0.05 1.08 99.59 3.92 0.58 MTF 07–10 Devine Canyon Tuff san 9.63 0.05 0.55 94.93 9.61 0.06 KCS 06-06 Alkali Buttes san 7.26 0.05 0.66 100 7.25 0.05 MTF 07–49 Bug Butte (top) plag 5.17 0.09 1.65 98.57 5.10 0.10 MTF 07–37 Spodue Mt (top) plag 5.77 0.07 1.19 100 5.81 0.12 MTF 07–58 Drake Peak (Trf) plag 8.96 0.06 0.64 95.84 8.95 0.13 MTF 07–52 Quartz Mountain Pass bt 6.79 0.04 0.58 80.32 6.81 0.08 MTF 07–30 Cougar Peak (top) plag 6.87 0.07 1.06 55.55 6.94 0.08 MTF 07–29 Dome south of Cox Flat san 8.05 0.04 0.52 99.84 8.07 0.06 MTF 07–14 Hagar Mountain (top) plag 5.67 0.16 2.83 100 5.73 0.28 MTF 07–53 Fergason Mountain plag 6.16 0.04 0.62 99.66 6.16 0.06 MTF 07–34 Bald Butte san 17.53 0.08 0.45 97.12 17.54 0.10 MTF 07–26 Drum Hill plag 17.30 0.09 0.53 98.71 17.33 0.11 MTF 07–57 Twenty Mile Creek san 15.03 0.08 0.56 100 15.04 0.09 MTF 07-02 Pine Mountain WR 2.75 6.25 0.03 0.52 43.67 6.25 0.04 aIn all cases, the plateau age and inverse isochron age are within 2 of each other and the plateau age is preferred. See Figure 4 for age spectra from each sample and Table S1 for sample location. bWR ¼ whole rock minicore, san ¼ sanidine mineral separate, bt ¼ biotite mineral separate, plag ¼ plagioclase series mineral separates with whole rock K2O values given. striking normal faults with offsets generally <10 (Figures 3a and 3b), some of which were dated in m[Lawrence, 1976] (Figure 2). Large range- this work (Table 1). bounding normal faults of the Basin and Range gradually decrease in topographic and structural 1.2. Geochronology and Geologic offset northward into the HLP (Figure 2). Although subtle, we use this preexisting, long- Background of the HLP-NWBR standing physiographic division to separate igne- [8] The volcanic rocks younger than 12 Ma in the ous rocks of the HLP from those of the NWBR. HLP and NWBR make up a basalt-rhyolite bi- While we use the physiographic boundaries for the modal province that consists of widespread, thin HLP in figures, the HLP volcanic province extends basalt lava flows and rhyolitic domes, lava flows further east to include the Duck Butte Eruptive and ash-flow tuffs, intercalated with tuffaceous Center and Jordan Craters. sediments. The basalts are dominantly low-K high-alumina olivine tholeiites, with the most [7] The NWBR (Figure 1) is that part of the Great primitive resembling mid-ocean ridge basalts Basin within Oregon, occurring between Steens [Hart et al., 1984]. The basalts do not exhibit any Mountain and the Cascade Arc, south of the HLP. age-progressive behavior, in contrast to the rhyo- The NWBR is cut by middle Miocene and lites, but do seem to focus in time to the axis of younger Basin and Range style block faults [Pez- the HLP [Jordan et al., 2004]. Basalt eruptive ac- zopane and Weldon, 1993] with scarps as high as tivity was concentrated in up to four pulses, with 2000 m (e.g., Steens Mountain, Abert Rim). the largest regional pulse around 7.5–8 Ma, and There are also diffuse northwest-striking faults, the most recent pulse of 50,000 years located at similar to the Brothers Fault Zone faults, com- either end of the HLP volcanic province, at Dia- prising the less documented Eugene-Denio Fault mond and Jordan Craters and around Newberry Zone (Figure 2). Some vent complexes (e.g., Volcano. Four Craters, Buttes of the Gods) mimic the regional NW to WNW fabric of these low-relief [9] Rhyolites of the HLP-NWBR volcanic prov- faults but there are not any vent alignments that ince erupted in an age-progressive manner, begin- parallel the larger offset, generally NNE striking ning in the east at around 10–12 Ma and younging ‘‘Basin and Range’’ faults. Compared to the HLP to the west-northwest, with the youngest eruptions there are more exposures of the older (>12 Ma) (<1 Ma) near Newberry Volcano. We exclude silicic volcanic rocks in the NWBR due to the Newberry Volcano (<0.6 Ma) from the HLP as it greater structural relief [cf., Scarberry, 2007] has previously been associated with Cascades

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volcanism [Donnelly-Nolan et al., 2008; Jensen son et al., 1990; Bestland and Retallack, 1994b] et al., 2009; Graham et al., 2009]. This west- but also within the HLP-NWBR (Figure 3a). northwest-younging age progression has posed a Along the northern margin of the HLP volcanism tectonic puzzle in that it propagates in the opposite at Hampton Butte [Iademarco, 2009], Bear Creek direction to the age progression of the rhyolites Butte and Powell Buttes [Evans and Brown, 1981] that define the Yellowstone-Snake River Plain are similar in age and composition, and they have trend [Pierce and Morgan, 1992], and nearly per- been ascribed to the John Day Formation. Some- pendicular to North American plate motion. Jor- what younger volcanism from 22 to 20 Ma is dan et al. [2004], who link the two parallel age- exposed along faults in the NWBR. This includes progressive trends of the HLP and NWBR to form the Coleman Hills [Scarberry et al., 2010], Rabbit a single province, term the entire suite the ‘‘HLP Hills [Scarberry, 2007], and Hart Mountain trend’’, which we prefer, in contrast to Humphreys [Mathis, 1993] which are similar in composition et al. [2000] and Xue and Allen [2006] who refer but not yet assigned to the John Day Formation. to the trend as the ‘‘Newberry hotspot’’ and ‘‘New- [13] The Steens Basalt lava flows, with a volume berry trend’’ respectively. up to 60,000 km3 extend over the eastern part of [10] More than 60 rhyolitic dome complexes dot the study area and into Idaho, Nevada and Cali- the region (Figure 3). While some are clearly mono- fornia [Carlson and Hart, 1987], and are contem- genetic, many are larger, more complex, polyge- poraneous with regional silicic eruptions from netic dome complexes like Glass Buttes or Juniper 17.5 to 15 Ma [Brueseke et al., 2008; Coble and Ridge. They are elongate in a northwesterly direc- Mahood 2012]. Isolated silicic domes 17.5–15.3 tion, parallel to the grain of the Brothers Fault Ma are exposed as kipukas both within the HLP Zone, and have internal age trends that young to the (e.g., Horsehead Mountain, Little Juniper Moun- west.Manyofthesedomesaredatedinthiswork tain, and Jackass Butte, Jordan et al. [2004]) and (Table 1) and many more were dated by Jordan et NWBR (e.g., Drum Hill and Bald Butte, Table 1; al. [2004] or reported in Fiebelkorn et al. [1983]. Figure 3b). The early to middle Miocene Straw- Table S1 (supporting information)1 contains a sum- berry Volcanic Field and various ash-flow tuffs mary of ages from silicic rocks of the HLP-NWBR. (e.g., Sucker Creek, Littlefield, Dinner Creek: Walker and MacLeod [1991]) crop out northeast 3 [11] Three large 100–300 km dense rock equiva- of the HLP (Figure 3b). lent (DRE) ash-flow tuffs had sources in the Har- ney Basin and crop out extensively (Figure 3d) [14] Finally, we do not include rocks of the including the Devine Canyon Tuff (DCT), Prater Deschutes Formation (7.4–4.0 Ma) which lies pri- Creek Tuff (PCT), and Rattlesnake Tuff (RST). marily north of Bend and consist predominantly Only the RST has been studied in detail [e.g., of pyroclastic flow deposits and volcaniclastic Streck, 2002; Streck and Grunder, 1995, 1997, sediments shed from the High Cascades [Smith et 1999, 2008, 2012]. A few of the domes have al., 1987; Eungard, 2012]. The Deschutes For- vented smaller volume ash-flow tuffs that are still mation also contains isolated 7–6 Ma domes preserved and there are likely more, yet undocu- including the Steelhead Falls rhyodacite and mented, small volume <12 Ma tuffs in the HLP Cline Buttes rhyolite [Sherrod et al., 2004] (Fig- and NWBR. ure 3c), which are associated with High Cascades Arc volcanic activity [Taylor, 1981]. We also [12] There are three major episodes of Cenozoic exclude Newberry Volcano from the HLP vol- volcanic activity in the region that predate volca- canic province. nism of the HLP-NWBR. The Clarno Formation (54–40 Ma) crops out to the north and west of the HLP in the Blue Mountains Terrane (Figure 1) to 2. Methods thicknesses >1800 m [Walker and Robinson, 1990; Bestland and Retallack, 1994a] and likely [15] Some samples were selected from undated si- underlies the HLP. The John Day Formation licic centers to better characterize the age progres- (38.5–20 Ma) crops out to thicknesses >1000 m, sion of the HLP-NWBR while other samples were primarily to the north of the HLP-NWBR [Robin- chosen for analysis to reduce the uncertainly or confirm the accuracy of previous K-Ar dates. Gen- erally, new 40Ar/39Ar dates have at least an order 1Additional supporting information may be found in the of magnitude better precision compared with pre- online version of this article. viously reported K-Ar dates. Additionally,

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samples with very high Rb/Sr ratios were preferen- from Pine Mountain yielded a six-step plateau tially targeted so that with the more accurate ages, concordant to the isochron age but with only 44% we could achieve more accurate initial radiogenic of the 39Ar released, and one sample from Dead isotopic values [cf., Ford, 2012]. Samples for this Indian Mountain failed to produce a reliable crys- work are a subset of samples selected for regional tallization age. Age spectrum plots are shown in geochemical analyses [cf., Ford, 2012]. Figure 4. Error (external) is reported to 2 for all analyses, and in nearly all cases, errors are <2% [16] Ages for nineteen rhyolitic samples, primarily of the calculated age and often <1%. Some ages from the NWBR, were determined by the of previously dated units changed significantly. 40Ar/39Ar incremental heating method at the Noble For example, the age of the Drake Peak rhyolite Gas Mass Spectrometry Laboratory at Oregon State has been updated from 14.7 6 2Ma[Wells, 1979; University. Samples were prepared as either min- Fiebelkorn, et al., 1983] to 8.96 6 0.06 Ma and eral separates (e.g., plagioclase, sanidine, biotite) or the age of Quartz Mountain from 7.8 6 0.4 whole rock minicores. Cores were prepared from Ma [Fiebelkorn, et al., 1983] to 6.79 6 0.04 Ma fresh, nonhydrated obsidian or from unaltered, ho- (Table 1). In all cases, precision was improved mogeneous, devitrified whole rock. Argon extrac- over previous K-Ar dates, sometimes significantly tion and analysis, using either a Heine low-blank, as exemplified at the Connelly Hills with a K-Ar double-vacuum resistance furnace (whole rock date of 6.35 6 0.65 Ma [Fiebelkorn, et al., 1983] cores, n ¼ 5) or a Merchantek integrated 10W CO 40 39 2 compared to our Ar/ Ar date of 5.59 6 0.05 Ma continuous fire laser (mineral separates, n ¼ 14), (Table 1). Our result for the DCT is very similar to followed the methods described by Duncan and those previously reported using 40Ar/39Ar total Hogan [1994], Duncan et al. [1997], and Duncan fusion analyses of single crystals. We report an and Keller [2004] using a Fish Canyon Tuff biotite age of 9.63 6 0.05 Ma based on a 10-step heating (FCT-3) monitor age of 28.02 6 0.16 Ma [Renne et plateau (Figure 4) recovering 94.9% of the radio- al., 1998]. Irradiation was at the Oregon State Uni- genic argon which is only slightly less than the versity 1 MW TRIGA experimental reactor facility. weighted mean total fusion date of 9.74 6 0.04 Ma The neutron flux was calculated from analyses of returned using 9–10 single-grain analyses [Jordan the FCT-3 standard. The total decay constant ( ) tot et al., 2004]. Three obsidian whole rock minicores incorporating the 40K 40Ar and 40K 40Ca (ec,p) (B-,n) were analyzed, viz: Glass Buttes ‘‘A’’ decays was 5.543 1010 yr1 as suggested by (5.79 6 0.04 Ma), Glass Buttes ‘‘E’’ (6.49 6 0.05 Steiger and Jager [1977]. Data reduction was done Ma) and Cougar Mountain (4.34 6 0.05 Ma) with the ArArCALC v2.2 software package [Kop- (Table 1). These samples released the majority of pers, 2002]. External errors (2) incorporate errors their 39Ar during one heating step, generally on decay constants, analytical errors (regressions between 1075C and 1250C, even using heating on peak heights versus time) and errors in fitting steps as small as 25C (Figure 4). measurements of monitors (J values), as suggested by Koppers [2002] and Min et al. [2000]. [19] The vast majority of the new ages from this study fall within the 12–0 Ma range which largely [17] We use the plateau age preferentially over the confirm earlier work. We also report some new isochron age in all cases because age uncertainty ages that do not fit the 12–0 Ma westward age is lower, resulting from generally low dispersion sweep of the HLP-NWBR volcanic province. We of step compositions in isochron plots. We define identify previously unknown middle Miocene vol- a plateau as five concordant, sequential steps com- canic centers at Bald Butte (17.53 6 0.08 Ma) and prising at least 50% of the total argon released. Drum Hill (17.30 6 0.09 Ma) (Figure 3b and Table Concordant plateau and isochron ages indicate a 1). We report a new age of 6.25 6 0.03 Ma (Table trapped argon component (initial composition) 1) for rhyolite located at the top of Pine Mountain, that is within error of atmospheric composition. and we believe this to be a reliable crystallization age despite the plateau containing only 44% of the total Ar released. 3. Results 3.2. Erupted Volumes 3.1. Age Determinations [20] While some estimates of the volumes of spe- 40 39 [18] Of the 19 samples dated, 17 yielded Ar/ Ar cific units or flows in the HLP have been calculated age spectra plateaus concordant with sample iso- [e.g., Streck and Grunder, 1995], there has been no chron ages (within 2) (Table 1). One sample comprehensive reporting of the volcanic volumes

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Figure 4. New age determinations for the HLP and NWBR. Eighteen 40Ar/39Ar age plateaus from the current study, 15 of which are from <12 Ma samples from the NWBR or HLP with one of these (Pine Mountain, 6.25 Ma) ascribed to the Deschutes Formation (see text and Table 1). The remaining three samples fall within the ‘‘Steens-age’’ time slice from 18 to 15 Ma (Figure 3b). Dashed lines on the diagrams indicate the heating steps that were integrated into the age calculation, the width of each box is the percent of total 39Ar released for that temperature step, and the height of each box is the 2 uncertainty for that heating step.

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Table 2. Estimated <12 Ma Volcanic Volumes Within the at least as great as those for the RST. Large tuffs HLP and NWBRa (over 100 km3) do not occur in the NWBR or west- Minimum Maximum ern HLP and all large tuffs are older than 7 Ma. 3 3 Location (km ) (km ) 3 [22] The smaller tuffs of the region are <40 km Deschutes formation negligible 100 each and as small as 1 km3 and source areas are This study, 12–0 Ma HLP 2000 2500 and NWBR often inferred based on location and age (reference Newberry related, 800 1000 Figure 3 for many of the following locations and including basalts Table 1 or Table S1 for additional ages). Frederick Total 2800 3600 Butte is the likely source of the Tuff of Espeland aAdditional volumes of the Deschutes and some Newberry related Draw [Fiebelkorn, et al., 1983; Walker,1974; flows extend outside the physiographic HLP and NWBR and are not Johnson, 1998], which has been correlated with included here. Total eruptive volumes considered for this study (see 40 39 text) are 1000–1250 km3 of basaltic, 1000–1250 km3 of rhyolitic, and the Hampton Tuff ( Ar/ Ar age of 3.80 6 0.16 100 km3 of intermediate composition volcanic rocks. Ma) by Iademarco [2009]. The maximum volume of the Hampton Tuff is estimated to be 40 km3 for the <12 Ma HLP-NWBR system. Our total vol- [Tucker et al., 2011]. A number of smaller tuffs ume estimate for volcanism <12 Ma in the study have unidentified sources. The Peyerl Tuff, located area is 2000–2500 km3 (Table 2) which contains on the margins of and within maars of the Fort subequal volumes of basalt and rhyolite and a minor Rock Basin, may be associated with an inferred amount of intermediate compositions. Although caldera structure at Wart Peak and Bald Mountain basalts, rhyolite domes and tuffs are widely exposed [Fiebelkorn, et al., 1983; MacLeod, et al., 1976]. (Figure 3), estimating the thickness and thus total We estimate the volume to be between 5 and 20 volume of these 12–0 Ma volcanic products is diffi- km3 (DRE). The peralkaline tuff of Wagontire cult. Cross sections through the volcanic units are Mountain, mapped by Walker and Swanson exposed only in substantial fault scarps, mainly in [1968], has an approximate volume of 1 km3 the NWBR, and in a few rare canyons in the HLP. (DRE) and a postulated source just west of Wag- Logs from a few energy exploration drill holes indi- ontire Mountain but it has not been dated. The cate that intercalated sediments and ‘‘Miocene and Buckaroo Lake Tuff has a 40Ar/39Ar age of younger’’ undifferentiated volcanic rocks extend to 6.85 6 0.04 Ma, overlapping within error of nearby depths of >1.5 km within the Harney Basin in the Elk Mountain [Jordan et al., 2004] but no source HLP [Milliard, 2010] and >3.5 km west of Lake- has been identified. The Silver Creek Welded Tuff view in the NWBR [Popenoe, 1961; Newton et al., may be sourced in the Yamsay Mountain area 1962]. Recent geophysical studies [Cox, 2011] indi- [Hering, 1981] but MacLeod et al. [1976] dis- cate that Cenozoic intercalated sediments and vol- agreed based on a K-Ar age of 6.77 6 1.10 Ma, canic rocks may be >7 km thick in the eastern which is older than other domes in the area. We Harney Basin, which is likely the locus of calderas estimate the volume of this tuff to be between 5 for the Devine Canyon and PCTs. Field work and and 20 km3 (DRE), assuming a Yamsay Mountain previous studies (see later) have indicated that the source. The Plush Tuff, exposed at Hart Mountain, 12–0 Ma volcanic rocks generally make up only a is undated and has a possible source near Beatys thin veneer where exposed, and while the Harney Butte [Mathis,1993;Larson, 1965]. The volume Basin may have appreciable volumes of these estimate of the smaller tuffs is based on outcrop younger rocks, we consider it more likely that the coverage and thickness from previous reconnais- basin contains mostly sediments and volcaniclastic sance mapping and by using the approximations of rocks with only a limited volume of <12 Ma basalts Streck and Grunder [1995] to account for erosional and rhyolites. loss, air fall, and conversion to DRE, using the RST as an analog. The total volume of all of the 3.2.1. Rhyolite Volume tuffs is likely 800–850 km3. [21] We estimate the total volume of <12 Ma rhyo- 3 litic volcanism in the study area is 1000–1250 km , [23] To the rhyolite volume of the tuffs, we added the majority of which is represented by the three 150–400 km3 from the silicic domes, many of major ash-flow tuffs in the HLP. Streck and Grun- which are <1km3, and their associated ash fall der [1995] calculated the volume of the RST at 280 deposits. Some of the larger complexes may be as km3 DRE. The PCT and DCT (Figure 3) are esti- much as 50 km3. The 6.5–5.7 Ma Glass Buttes mated to be 100–150 and 200–300 km3 respectively complex (Figure 3) is 20 km long, 2–5 km wide [Green, 1973]. We chose the higher end of Green’s and while variable in thickness, rhyolite extends to estimate for the DCT as its extent and thickness are depths of at least 1 km in places, based on

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hydrothermal energy exploration drilling [Johnson flows, ranging in age from 7.9 to 4.0 Ma [Jordan et and Ciancanelli, 1984]. This yields a volume esti- al., 2004], each with distinctive flow thicknesses mate of approximately 25–50 km3 for the com- and morphologies, crop out in Dry River Canyon plex. The silicic rocks of Drake Peak (Figure 3), (HLP). These three flow stacks are separated by located in the NWBR, are likewise estimated to unconformities and make a composite thickness of have a volume of 25–50 km3 (based on cross sec- 100 m, representing one of the few thick expo- tions of Wells [1979]). Other centers are interme- sures of mafic lavas in the HLP. Multiple different diate in size, including the Duck Butte Eruptive models, some of which factor the greater aerial cov- Center, which is estimated to be 7 km3 [Johnson erage and greater thickness along the axis of the and Grunder, 2000] and Juniper Ridge (Figure 3), HLP and less coverage and thickness across the which is conservatively estimated to be 3 km3 NWBR, converge to yield a total volume of basaltic [Maclean, 1994]. The total volume of silicic and basaltic andesite volcanic rocks of 1000–1250 domes and flows is estimated to be 120–250 km3. km3, roughly equivalent to that of the rhyolites.

[24] The main sources of error in the volume esti- [26] The majority of the volume of intermediate mate of the rhyolites are uncertainties in thickness compositions (andesites and dacites) is associated and the unknown quantity of dispersed pyroclastic with Gearhart Mountain in the NWBR (Figure 3). fall deposits. Two very recent, local analogs provide Gearhart Mountain has a composite shield morphol- insight in to the amount of pyroclastic fall deposits ogy and is primarily andesite, with subordinate vol- that these eruptions might have produced. The 700 umes of basalts, dacites and rhyolites [Brikowski, C.E. eruption of the Big Obsidian Flow at Newberry 1983]. Yamsay Mountain (Figure 3), also in the Volcano (Figure 3) has a well documented associ- NWBR, contains a number of intermediate compo- ated plinian fall deposit of 0.1 km3 (DRE), or about sition samples in the literature, but the bulk of the 40% of the total eruption [Jensen, 2006]. Both the estimated 150 km3 shield volcano is basaltic ande- 950 C.E. and 1100 C.E. Little Glass Mountain and site [Hering, 1981]. Hagar Mountain, again in the Glass Mountain flows of the Medicine Lake Vol- NWBR, has an unknown volume of intermediate cano yielded about 10% by volume (DRE) of tephra rocks but a total volcanic volume likely <20 km3. [Heiken et al., 1978; Donnelly-Nolan et al., 1990]. Intermediate compositions in the HLP [cf., Mac- Using these eruptions as proxies for HLP-NWBR lean, 1994; Johnson, 1995] are over sampled with volcanism would add an estimated 12–100 km3 to respect to their volume and total volumes are not the volume of the silicic domes in the region. great, likely <20 km3 in total. Thus, the total volume of intermediate rocks (andesite and dacite) 3.2.2. Basalt and Intermediate Composition Volume from the HLP and NWBR is approximately 100 km3, about 5% of the total volume of volcanic [25] Most of the <12 Ma basalts and basaltic andesites in the HLP and NWBR are low-K, high-alu- rocks of for the <12 Ma time period. mina olivine tholeiites but subordinate calc-alkaline 3.2.3. Localized Events Excluded in This Volume and alkali basalts exist [cf., Hart et al., 1984; Jor- Estimate dan, 2002; Streck and Grunder, 2012]. Areal cover- [27] We do not include Newberry Volcano in our age is estimated to be 25,000 km2, roughly 1/2 of volume estimate of the <12 Ma HLP-NWBR and the study area, with a greater concentration of flows this shield has a volume of 800–1000 km3 along the axis of the HLP and few exposures east of [MacLeod et al., 1995]. We also exclude the 119.5 in the NWBR. High-alumina olivine tholeiite Deschutes Formation and Pine Mountain which flows are commonly 5 to 30 m thick [Hart et al., falls within the 12–0 Ma age range and crops out it 1984; Mathis,1993;Johnson,1995;Scarberry, the northwest portion of the physiographic HLP. 2007; Iademarco, 2009] where normal faults or can- We limit the volume related to the Deschutes For- yons expose cross sections of basalt. Individual flow mation in the HLP to 100 km3 (Table 2). fields commonly occur as compound stacks of several to a dozen flows with no evidence of erosion or soil development between stacks. Multiple stacks of 3.3. Geologic Map Updates flows, separated by tuffaceous sediments or volcanic [28] In Figure 3d, we provide some minor updates rocks, are exposed for 100 m at Hampton Buttes to the state geologic map [Walker and MacLeod, [Iademarco, 2009], in drill core for 40matGlass 1991] that were discovered during field work. Buttes [Johnson, 1984], for 55 m at Abert Rim These include the basalt of Dry Mountain [Iade- [Scarberry et al., 2010] and for 50 m at Burma marco, 2009] and three nearly circular, flattened Rim [Jordan et al., 2002]. Three distinct stacks of dome-like formations which from a distance

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Figure 5. Map with a stretched color ramp of interpolated ages over much of the study area, with HLP and NWBR silicic centers in purple. Known ages (Table 1, Table S1) <12 Ma represented (see key). Coloring indicates age and was created by an IDW stretched color ramp. White space indicates that there was not sufficient data for interpolation. Green boxes are towns (B ¼ Burns, L ¼ Lakeview), heavy dashed line outlines physiographic HLP (Figure 1) and black lines are major roads. Proposed location for the RST source from Streck and Grunder, [1997] and proposed locations of vents, based in part from this map, field evidence and Grun- der (unpublished) for other tuffs (Hampton, Prater Creek, and Devine Canyon) shown with dashed circles. 40Ar/39Ar age for the Hampton Tuff is from Iademarco [2009]. The following points were removed for this interpolation: The Deschutes Formation-aged 6.25 Ma Pine Mountain (Figure 3c) and 6.8 Ma China Hat and the 2.89 Ma Iron Mountain (shown in yellow text). suggest viscous flow. The latter three are a consid- province established by MacLeod et al. [1976] and erable distance from reasonable roadways and modified by Jordan et al. [2004] and expand the were originally mapped as rhyolite [Walker and range of this age distribution to unequivocally MacLeod, 1991] but when field checked they were include the NWBR. We have applied an inverse determined to be basalt or basaltic andesite. distance (squared) weighted (IDW), stretched color ramp (IDW interpolation: Shepard [1968] and 4. Discussion Naoum and Tsanis [2004]) to the age determinations to create a color-coded map (Figure 5). This illustrates that the west-northwest younging pattern 4.1. Age Progressive Volcanism applies to silicic rocks in the HLP-NWBR as far [29] Our new ages increase the density of data, south as the California border, and that these rhyo- refine and support the westward-younging age pro- lites constitute a single, age-progressive volcanic gression of rhyolite centers in the HLP volcanic province that has been active over the past 12 Ma

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[30] Although the isochrons of Jordan et al. [33] The 6.25 6 0.03 Ma low-silica rhyolite at [2004] display the westward sweep of silicic vol- Pine Mountain crops out in an area where the age canism, our graduated interpolation has the trend would indicate rhyolitic volcanism be <1 advantage of not forcing a particular isochron Ma (Figure 3c). Pine Mountain is similar in age to through the data, therefore reducing the number of the low-silica rhyolite at Cline Buttes located apparent misfits, and shows the age progression as about 60 km away and the rhyodacite of Steelhead a more systematic sweep than as depicted by the Falls of the Deschutes Formation, which are isochrons. It should be noted that one limitation of related to Cascades volcanism [Sherrod et al., the IDW interpolation is the radial symmetry 2004] and we believe this to be the southeastern- [Watson and Philip, 1985] or ‘‘halo’’ effect cre- most exposure of this formation, although further ated by some isolated data points (e.g., the 9.5 Ma study is needed. A K-Ar age of 22.0 6 4Ma[Fie- K-Ar age in the eastern HLP). However, despite belkorn, et al., 1983] is reported from a sample on the proclivity for the IDW interpolation to high- the flanks of the mountain, indicating multiple epi- light anomalous ages, there are enough consistent sodes of volcanism older than the age progression data within the HLP-NWBR system to discern a are exposed at this location, one possibly correla- broadly WNW volcanic transgression. tive to the Deschutes Formation and the other to the John Day Formation. China Hat dome, located [31] Excluded from this analysis are two older epi- just south of Pine Mountain, has published K-Ar sodes of silicic volcanism that crop out in the area, dates of 6.3 6 1.1 and 7.4 6 0.8 Ma (Table S1; (1) the 38.5–20 Ma, region-wide events of Fiebelkorn et al. [1983]), indicating that there subduction-related calc-alkaline John Day volca- could be other Deschutes Formation rhyolites in nism [Robinson et al., 1990] and (2) silicic volca- this area. Neither of the Deschutes aged centers or nism coeval with the Steens Basalt lava flows Iron Mountain are included in the determination of episode, generally 17.5–15 Ma. Exposures of John the color ramp for Figure 5. Day-aged volcanism crop out at the margins of the HLP volcanism or in scarps created by Basin and [34] When combined, the HLP and NWBR show a Range faults within the NWBR [cf., Iademarco, 12–0 Ma sweep of volcanism across Oregon but 2009; Scarberry et al., 2010]. A few paleotopo- eruption rates vary across the province. Volumi- graphic highs of Steens-age felsic volcanic rocks nous tuffs (RST, PCT, and DCT) erupted only occur in both the HLP (e.g., Horsehead and Little from the HLP and only between 10 and 7 Ma, and Juniper Mountain, Jordan et al. [2004]) and in the accounted for well over half of the total volume of NWBR (e.g., Bald Butte and Drum Hill, Table 1; silicic volcanic rocks. A further difference Hawks Mountain, Brueseke et al. [2008]). Addi- between the two subprovinces is that HLP activity tional Steens-age felsic volcanic rocks lie east and occurred over the entire 12–0 Ma time span south of the <12 Ma HLP-NWBR volcanic prov- whereas the NWBR does not contain rhyolites ince described here. younger than about 5 Ma. Finally, the age color ramp map (Figure 5) reveals several gaps of >1 [32] Only two of the <12 Ma rhyolitic lavas do Ma in the rhyolite ages, or at least gaps in rhyolite not conform to the westward age sweep. One is exposures, within the HLP including the intervals Iron Mountain, which has an age of 2.89 6 0.16 from 9.6 to 8.4, 8.2 to 7.3, and 5.8 to 4.34 Ma. The Ma, but lies in an area of rhyolitic volcanism that NWBR has substantially fewer dated samples but is about 7 Ma [Jordan et al., 2004] (Figure 5). We there are no gaps in dated rhyolite exposures interpret this center to be part of the HLP, but it is >0.75 Ma. These larger gaps in rhyolite ages in clearly a postage-progressive rhyolite and may be the HLP may be due to lulls in rhyolitic activity or associated with a pulse of young basaltic activity may be a result of younger cover in basins or by that extends along the length of the HLP. Iron basalt flows. Each of these possibilities is dis- Mountain is analogous to post rhyolite-sweep cen- cussed later. ters in the eastern Snake River Plain that are asso- 4.1.1. Basal—Rhyolite Connection and Timing of ciated with younger basaltic volcanism (e.g., Big Volcanism Southern Butte, Cedar Butte, East Butte; McCurry [35] Inasmuch as a flux of basalt is required as et al. [2008]). In contrast, there are no silicic cen- either a thermal or material source for rhyolitic ters in the NWBR younger than the age progres- magma production [e.g., Hildreth, 1981; Huppert sion. The NWBR also lacks a distinct pulse of and Sparks, 1988; Nekvasil et al., 2000], the 12–0 younger (<3 Ma) basaltic volcanism although Ma basalts of the HLP-NWBR do not make an age there may be a few isolated young basalt flows. sweep comparable to the rhyolites and instead

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occur in episodes. We examine the link between [38] Another, less pronounced pulse of basaltic the timing of basaltic pulses and rhyolite produc- volcanism occurred province wide from 3 to 2 Ma tion, primarily in the HLP, first looking at cases [Jordan et al., 2004]. While extending the length where there may be a temporal link and then at of the HLP, this pulse is not represented in the periods where no such link is evident. In some NWBR and is smaller and more focused on the cases, basaltic magmas may be stalled in the crust axis of the HLP compared to the pulse at 7.5 (intraplated) if regional tectonics or crustal lithol- Ma. The smaller overall volume suggests a general ogy (density) are not conducive to eruption, simi- waning of the basaltic flux, but perhaps sufficient lar to the conditions inferred for some basalts of to produce rhyolite as indicated by the emplace- the Snake River Plain [McCurry and Rodgers, ment of the 2.8 Ma Iron Mountain. The closest ba- 2009]. In other cases, the related basalts may sim- salt of a similar age crops out 30 km away. ply be covered by younger volcanic rocks or, in Reduced rhyolitic activity at this time is also likely the case of the Harney Basin, located only in the a result of the crust becoming more refractory due subsurface due to basin subsidence. to continued injection (intraplating) of basaltic magma and mafic residuum from earlier rhyolite [36] Basaltic magmas erupted widely across much production [Ford and Grunder, 2011]. While there of the HLP from 7.8 to 7.5 Ma [Jordan et al., are no young basalts in the NWBR, there are also 2004]. This pulse was soon followed by volumi- no rhyolites <4 Ma either. nous rhyolite volcanism within the HLP from 7.3 to 6.9 Ma, including many domes and the 280 km3 [39] A correlation between basaltic volcanism and RST at 7.1 Ma [Streck and Grunder, 1995; Jordan the older set of voluminous tuffs (PCT at 8.4 Ma et al., 2004], representing approximately a quarter and DCT at 9.63 Ma), which account for approxi- of the total volume of silicic magmatism (Figure mately a quarter of the rhyolitic volcanism, is 3d). In part, the basaltic flows and voluminous more difficult to establish. Basalts of a broadly RST may well have covered rhyolitic lavas similar age are associated with the 10.5 Ma erupted coeval with basaltic lavas or there may emplacement of the Duck Butte Eruptive Center have been a thermal lag between basalt and rhyo- [Johnson and Grunder, 2000], Malheur caves area lite activity [Jackson et al., 2003]. Furthermore, [Jordan et al., 2004], and the 12–9 Ma ‘‘Southern there are limited basalts in the NWBR but still Harney Basin Group’’ [Brown et al., 1980] which some silicic centers within the NWBR that fall is south of the proposed source location of the within this age range (Figure 5). The spatial rela- DCT (Figure 3d). In addition to these basalts, the tionships of basalt to rhyolite eruption and the more widely distributed 12.5–10 Ma Keeney delay between the two, however, demonstrates Sequence flows crop out <45 km east of the that basalt eruption is not directly coupled to rhyo- source of the DCT [Hooper et al., 2002; Camp et litic volcanism, as the geographic extent of the al., 2003]. Only one basalt of this age crops out in 7.8–7.5 Ma basalts are not accompanied by a simi- the NWBR [Scarberry et al., 2010]. Other basalts lar distribution of rhyolites or, alternatively, the that might have been erupted near the time of the 7.3–6.9 Ma flare-up of rhyolitic volcanism is not PCT and DCT eruptions are relegated to the sub- accompanied by widespread basaltic volcanism. surface in the Harney Basin, as basalt-sediment Regardless, there is a pulse of basaltic activity that interbeds extend to depths >1000 m in many places just predates rhyolitic volcanism including a large- [Newton et al., 1962; Milliard, 2010]. Much of this volume tuff. sediment was deposited after the rapid onset of subsidence in the basin starting at approximately 10 [37] Jordan et al. [2004] also postulated a small Ma and ending or slowing significantly by the time pulse of basaltic volcanism between 5.9 and 5.3 of the emplacement of the 7.25 Ma Drinkwater ba- Ma which is restricted to the western part of the salt [Milliard, 2010]. Additionally, the voluminous HLP-NWBR border. This increased activity is co- PCT and DCT and younger lava flows and sediment eval with or slightly older than nearby rhyolitic may cover many basalt flows that are related to this volcanism (e.g., Hagar Mountain, North Connelly period. An alternative which cannot be disproved Hills, Yamsay Mountain; Figure 3d and Table 1). with current information is that due to different tec- Due to the paucity of basalt ages within the tonic stresses at this time, basalts associated with NWBR, it is not clear if mafic volcanism of this these rhyolites did not erupt to the same degree as age extends throughout the province and relates to younger basalts. It is unclear if basalt ‘‘bedrock’’ the 6–5 Ma rhyolitic volcanism in the NWBR, so encountered in the well logs at depth is part of the the correlation is weak. middle Miocene Steens Basalt lava flows or a

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younger, ‘‘HLP-aged’’ flow. Either there are no ba- 4.1.2. Subsidence of the HLP and Effects on saltic flows associated with voluminous rhyolitic Rhyolite Outcrop Distribution eruptions of this time, or they are only sparsely [42] The geomorphic expression of the HLP is an exposed at the surface. elongate trough with generally muted topography, similar to the Snake River Plain. Areas with few [40] The most recent pulse of HLP-wide basaltic rhyolite domes exposed reflect apparent age gaps volcanism spans the past 50 ka. While much of in volcanic activity in the HLP. The Harney Basin this young basaltic volcanism is centered around has a gap from 9.6 to 7.3 Ma with only one dome Newberry Volcano and to the east of Newberry exposed within that range, and the area south of (e.g., Potholes, Devils Garden flows) [Jordan et Frederick Butte has a gap from 5.8 to 4.34 Ma al., 2004; Jenson, 2006], some young volcanism (Figure 3d and 5). The paucity of rhyolitic lavas occurs well to the east in the HLP volcanic prov- between 9.6 and 7.3 Ma in the Harney Basin we ince, including Diamond Craters [Russell and attribute to cover by younger volcanic rocks and Nicholls,1987;Sherrod et al., 2012] and Jordan sediments resulting from subsidence of the HLP, a Craters [Otto and Hutchison,1977;Hart and consequence of persistent injection of basalts into Mertzman, 1983]. No young basalts of this age the crust. The paucity of rhyolites of 5.8–4.34 Ma are present in the NWBR based on flow mor- age is partly a result of cover by younger volcanic phologies and age determinations. Except for rocks (6 limited sediment), especially on the Holocene rhyolites at Newberry Volcano (e.g., flanks of Newberry Volcano, but may also reflect a Big Obsidian Flow), there are no rhyolites asso- change in regional tectonics. These processes are ciated with this youngest pulse of basaltic activ- briefly explored later. ity. We attribute this to a combination of [43] The dips into the basin on tuffs along the insufficient flux of mafic magmas and a crust northern margin of the Harney Basin increase with that has been made too refractory for partial increasing age, so that the 9.6 Ma DCT has a dip melting [Ford and Grunder, 2011], or the ther- of 6, the 8.4 Ma PCT dips at 3 and the 7.1 Ma mal lag between basalt and rhyolite activity RST dips <1 [Milliard, 2010]. The changes in [Jackson et al., 2003]. dip were not caused by any uplift in the adjacent Blue Mountains province within the past 10 Ma [41] In all of the above cases, there is either a [Walker and Robinson, 1990]. Broad, gentle folds spatial or temporal decoupling between the erup- located along the southern margin of the Harney tions of basalts and rhyolites. For the 7.8–7.5 Ma Basin in RST-aged volcanic rocks are in-filled pulse, regional tectonic events (e.g., initiation of with 6.1 Ma (K-Ar age) flat-lying basaltic flows the High Cascades, a change in Pacific Plate and plunge toward the center of the basin [Brown motion) likely triggered deformation and the et al., 1980]. widespread eruption of basalts that is strongly concentrated in the HLP, as discussed in Jordan [44] The Harney Basin region has experienced et al. [2004]. It is unclear if localization of downward crustal flexure or sag similar to that of crustal stresses created pathways for basaltic the Snake River Plain [McQuarrie and Rodgers, magmas to reach the surface or if increased melt 1998] with crustal loading due to mass transfer volumes in the crust reduced rock strength to from the mantle to the crust via intraplated basaltic facilitate faulting [Rosenberg et al., 2007]. At magmas, diking and volcanism, with additional this time, NWBR has a kinematic link to the mass added by basin sedimentation. Even shallow Brothers Fault Zone in the HLP [Trench et al., dips on units to the north and south of the basin 2012]. By the time of the 5.9–5.3 basaltic pulse, could easily manifest multiple kilometers of Trench et al. [2012] propose a decoupling of the crustal downwarping on the scale of the study kinematic link between the HLP and NWBR, area. The one dome that does crop out within this resulting in an independent slip history for the age range is Burns Butte, located on the northern HLP and a reduction in deformation rates. This edge of the Harney Basin (Figure 3d), where sub- could be the reason that the 3–2 Ma and <50 ka sidence would be minimal. Recent geophysical basaltic pulses are relegated to the HLP whereas studies [Cox, 2011] indicate volcanic rocks and the 7.5 Ma pulse covered a greater area. The sediments may extend to as deep as 7 km in the mantle-derived injections that drive silicic mag- Harney Basin, at least 2 km deeper than other pla- matism are decoupled from the basaltic erup- ces in the HLP or Basin and Range. Well logs tions, which reach the surface based on local or from the eastern part of the Harney Basin indicate regional transtension. 0.75 to more than 1.5 km of sediment [Milliard,

2849 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175

2010]. The timing of subsidence coincides with et al., 2004] and the North American Plate moves voluminous rhyolitic eruptions, and subsidence S40Wat26 km/Ma [Gripp and Gordon, 2002]. wanes after the focus of silicic volcanism moved Simple vector addition of North American plate westward. motion and the HLP age-progressive rhyolitic volcanism trend very closely matches the direction of [45] The lack of exposed silicic volcanism from asthenospheric flow, as recently determined by 5.8 to 4.34 Ma in the western HLP likely has a dif- shear wave splitting (SKS phases) delay times ferent cause. There is an overall decline in silicic [Long et al., 2009] (Figure 6). The calculated volcanic volume in the HLP-NWBR starting at asthenospheric flow is 53 km/Ma, which is con- this time. Activity ceased in the NWBR by 5 Ma, sistent with the magnitude of the S wave delay and in the HLP, rhyolites are constrained to a nar- times, some of the largest recorded, indicating row band of small-volume domes on the eastern very strong anisotropy and alignment in the as- flank of the Newberry shield <3.7 Ma. There are thenosphere [Long et al., 2009]. Additionally, no large dome complexes erupted on the HLP after young basalts from the HLP have He/4He values the 5.7 Ma, 25–50 km3 Glass Buttes. This would of 8.8–9.3 R , which are within the range of mid- correspond to a significant decrease in basalt addi- A ocean basalts and do not indicate a plume compo- tion, and we consider it unlikely that the relatively nent [Graham et al., 2009]. small amounts of mass added to the crust in this area could cause downwarping. [48] Many subduction zones are dominated by [46] Extension via Basin and Range faults has trench parallel flow (toroidal edge flow) or show propagated in episodic steps westward (and north- heterogeneous flow vectors [e.g., Long and Silver, ward) across the NWBR over at least the past 10 2008] but the flow vectors under the HLP are Ma [Scarberry, et al., 2010]. This westward propa- nearly unidirectional (Figure 6d) [Long et al., gation into the Cascades corresponds with an 2012]. Other workers using three-dimensional increase in volcanic activity within the arc in cen- strain modeling [Druken et al., 2011; Long et al., tral Oregon, just prior to 4.5 Ma [Hughes and Tay- 2012] have recently shown that the downdip only lor, 1986]. This model is consistent with the motion (Figure 7) on the Cascadia slab is insuffi- marginward migration of basalt or bimodal basalt- cient to create the mantle wedge flow observed rhyolite volcanism throughout the entire Basin and under Oregon and thus a combination of trench Range province and the general westward sweep rollback, slab steepening, and back-arc extension of rhyolitic volcanism in the northern Basin and is required to produce the seismic anisotropy Range from 12 to 5 Ma [Luedke and Smith, 1984; observed. Downdip motion alone only produces Fitton, et al., 1991]. This transfer and reorganiza- weak corner flow and little upwelling from the tion of crustal stresses to the axis of the active arc mantle. may have greatly reduced either the formation of rhyolitic magmas or their ability to reach the sur- [49] Geodynamical models from Long et al. face in the HLP and NWBR. Also, Pliocene and [2012] require modest back-arc extension to draw younger basalt coverage is more extensive in this material from depth to give the thermal modeling area than in other parts of the HLP, especially as results. But, they do not separate trench rollback one approaches the Newberry shield, and this may induced anisotropy from that related to slab steep- have covered some small-volume domes. ening. Long et al. ’s [2012] results also show that mantle flow is focused inboard (northward) of the slab edge a considerable distance in the slab roll- 4.2. The HLP-NWBR Age Progressive back, steepening and backarc extension models. Trend and Asthenospheric Flow, Other workers have shown that the southern por- A Unifying Model tion (that portion under Oregon and northern Cali- fornia) of the Cascadia slab is coherent and is not [47] An unresolved question is, what has caused fragmented until it reaches the transition zone the age progressive volcanism in the HLP and (>410 km) [Roth et al., 2008; James et al., 2011] NWBR? We propose that the volcanism in the and thus a shallow fragmented slab does not pro- HLP and NWBR is a result of Cascadia slab plate- duce the mantle upwelling required in this region. driven asthenospheric flow and does not require a mantle plume. Surface expression of HLP rhyolitic [50] The southern Cascadia slab is subducting at a volcanism moves approximately N20Wat33 steeper angle than the portion under Washington km/Ma, from 12 Ma until 4 Ma (Figure 6) [Jordan State (Figure 7). This is based on the trench to arc

2850 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175

Figure 6. Resolving asthenospheric flow and plate motion to create the HLP volcanic trend. (a) Four time slices at 10.5 (Duck Creek Butte), 7.1 (RST), 3.9 (Frederick Butte and Hampton Tuff), and 0 Ma (Newberry Volcano, present day) showing the posi- tions of subsequent eruptions with the motion of the North American Plate (26 km/Ma) Colors for eruptions from previous time slices are muted. Figure is to scale indicating westward asthenospheric flow rate is generally uniform, averaging 53 km/Ma from 10.5 to 4 Ma. (b) A simplified geologic map of the HLP showing only the four age-progressive eruptions discussed (compare to Figure 3d). (c) Surface expression of HLP rhyolitic volcanism moves WNW at 33 km/Ma [after Jordan et al., 2004] but the rate slows at 4 Ma (see text). The four episodes discussed are plotted (NB ¼ Newberry Volcano). Dotted box in Figure 6a indicates where volcanism would plot if the HLP volcanic trend rate had remained at 33 km/Ma, approximately 25 km south of South Sister Volcano (SS) in the Cascades. (d) A histogram showing shear wave splitting (SKS phases) delay times for the HLP [Long et al., 2009] with black arrow from this calculation of asthenospheric flow from Figure 6a. distance, if volcanism occurs at the same depth band, which results in the HLP track. This focused above the slab along the arc [cf., Hildreth, 2007], flow results in a greater flux of basaltic magmas to the location of the shallow slab based on earth- drive silicic magmatism and resulted in intraplated quake locations and depths [McCrory et al., 2012] basalts and mafic residuum, a process we term and P wave tomographic results [Roth et al., ‘‘basaltification.’’ This basaltification, especially 2008; Pavlis et al., 2012]. Slab steepening is also in the Harney Basin area, produced crustal flexure suggested as the southern portion of the slab is and helped shape the overall topography of the unsupported [e.g., Zandt and Humphreys, 2008]. region. Partial melts of the increasingly mafic crust Slab steepening could also result in enhanced hori- in the HLP produced the iron-rich rhyolites zontal mantle alignment. whereas rhyolites of the NWBR are lower in iron [Ford, 2012]. This combination of focusing of [51] The progressive steepening of the slab from asthenospheric flow resulting from the downgoing north to south may focus the increased astheno- slab and possibly lithospheric permeability in the spheric counterflow or upwelling to a more narrow HLP results in the enhanced volcanism along the

2851 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175

Figure 7. Schematic models (nearly to scale and with approximately no vertical exaggeration) of the processes that drive mantle upwelling and counterflow as a result of the subducting Cascadia slab. Plate motions are shown with black arrows and mantle flow with red arrows. Red triangles are the current day locations of Cascade volcanoes and the Cascades, HLP, and NWBR physiographic provinces are outlined (Figure 1). The position of the trench and transform faults (red dotted line) are also shown. (a) Limited corner flow associated with downdip motion only, before trench rollback commenced. (b) Slab rollback initiates a much stronger mantle flow as North America continues to advance on the trench. (c) The addition of slab steepening due to the southern edge of the slab being unsupported can further enhances this flow, which is concentrated under the HLP. Differing extensional rates in the continental slab (crustal factors) may also play a role in the volcanic expression. The current geometry of the Cascadia slab is shown in orange. (d) Simplified version of Figure 7c showing the current Cascadia slab only. Angles given are approximate and taken from McCrory et al. [2012] and Roth et al. [2008] and the slab steepens markedly at the volcanic front (100 km depth) [Pavlis et al. 2012].

HLP. While there is very little crustal extension in tial melting of the mantle, a changing astheno- the HLP [Meigs et al., 2009; Trench et al., 2012], spheric source composition or interaction with a it is still greater than that in the adjacent Blue more depleted mantle lithosphere [cf., Carlson Mountains (Figure 7). and Hart, 1987]. Modeling of thermal anomalies [52] We also observe that basalts trend to higher using trench rollback and extension in the backarc MgO/(MgOþFeO) through time [Ford, 2012], so also indicates that mantle temperatures should that younger basalts are hotter than older basalts, increase over time [Long et al., 2012]. In our regardless of E-W location in the HLP. This indi- model, slab rollback, back-arc extension, and the cates that the parental basaltic magmas to this sys- progressive steepening of the slab to the north tem might be changing through time, toward results in enhanced counterﬂow (Figure 7) and a hotter basaltic magmas and greater degrees of par- plume in not a necessary heat source. We favor an

2852 FORD ET AL.: AGE-PROGRESSIVE RHYOLITES OF OREGON 10.1002/ggge.20175

enhanced ﬂow in the mantle to generate the volu- required. Volcanism in the HLP and NWBR is metrically superior HLP while lithospheric proc- driven by this ﬂow and focused in the HLP due to esses may play a larger role in the related slab rollback, steepening and back-arc extension. volcanism of the NWBR, due to substantially greater amounts of crustal extension. Acknowledgments [53] If the HLP volcanic trend continued to move at 33 km/Ma, instead of appearing to slow at about [55] This work is part of the High Lava Plains Project, with 4 Ma, the current location of volcanic activity support provided by the NSF Continental Dynamics program would be at the apex of the Cascade range, through grant EAR-0506869 (ALG and RAD). We thank approximately 25 km south of South Sister vol- Layne Bennett and Darrick Boschmann for GIS assistance cano [cf., Hill and Taylor, 1989; Jordan et al., and the VIPER (Volcanology, Igneous Petrology and Eco- 2004] (Figure 6). The highest density of volcanic nomic Research) group at Oregon State University for discus- vents in the Oregon Cascades and very young sion. We would also like to thank Matt Brueseke, Gail (<40 ka) silicic centers are found in the Three Sis- Mahood and Rick Carlson for their insightful and thoughtful ters Volcanic Field [Guffanti and Weaver, 1988; reviews of this paper. Schmidt and Grunder, 2011], which is at the inter- section of the HLP trend and the Cascades graben. References

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